Blow molding extrusion head

ABSTRACT

An extrusion head for a blow molding machine wherein a movable plunger is slidably mounted in a distributor head which receives a melt at an inlet end of its channel and directs it along such channel to the terminal end of the channel while simultaneously directing portions of it along spiral radial paths to the lower circumferential edge of the distributor head to fill an accumulator chamber which also causes the plunger to recede facilitating the filling of the accumulator chamber. The channel as defined by the adjacent circumferential wall and the curvilinear contour on the distributor head defines a smooth integral flow channel that is curvilinear in cross section.

BACKGROUND OF THE INVENTION

The present invention relates to an extrusion device for use in blowmolding machines and more particularly to a new and improved extrusionhead having a plunger and a distributor head for feeding plasticflowable material into an accumulator for blow molding.

Extrusion-blow molding is done as a continuous extrusion process or asan intermittent extrusion process. The continuous extrusion process isbest suited for PVC (polyvinyl chloride) and other heat-sensitive resinsbecause the gentle uninterrupted flow reduces the chance for resinthermal degradation. In such continuous molding, the molding processused is either shuttle or rotary to accommodate the continuous flow. Theintermittent extrusion process is best used for polyolefin and othernon-heat sensitive resins and permits the use of simpler moldingmechanisms. The intermittent extrusion machines fall into threecategories: reciprocating screw, ram accumulator, and an extrusionaccumulator head systems. The reciprocating screw allows accumulation ofthe melt in front of itself and thence the screw is rapidly pushedforward, forcing the melt through a die head to form a parison. In thisinstance, the melt remaining in the screw during the forward movementthereof is subjected to heat and can cause problems of degradation iftried with PVC or other heat sensitive thermoplastic materials. The ramaccumulator allows the accumulation of the melt in an auxiliary cylinderand is extruded by a special ram. In this case, the disadvantage is thatthe melt that enters the accumulator first is the last to be extruded.The melt history for this system is not uniform. The accumulator headserves as an extruding die head type having a movable tubular plungerwhich moves as the accumulator chamber is filled directly from theextruder which has a first in/first out melt flow path. In a typicalaccumulator head, the melt enters from one side and divides as it flowsaround the plunger head, reknitting on the opposite side as it fills theaccumulator chamber. One problem encountered with this process and theuse of the equipment available is weld or knit lines. From arheologically correct design of melt distribution, it appears and showsa thickness distribution that because of orientation in the flow lines(knit-lines) generated where the divided melt stream comes together, itstretches more here than elsewhere and this leads to thin spots in theblown article. This stretching becomes more apparent when increasing thedegree to which the article is increased in size.

Reference is made to U.S. Pat. No. 4,422,838 which discloses anextrusion head with a body portion supporting a movable diverter plungerthat is located therein, such that as a melt flows into the body portionthrough an inlet aperture, the melt flows via a groove into a cavity orflared portion to move around a throttling lower edge. The gap betweenthe cavity and the surrounding wall is essentially uniform as seen inFIG. 4. Herein melt will flow up the groove as the plunger rises, whichmelt will not be extruded until after an accumulation. In addition, thedimension of the gap of the longitudinal flow channel is essentiallyconstant from the inlet end to the terminal end which results into anon-uniform flow beyond the tapered shoulder or throttling lower edge.Such restrictive throttling lower edge results in excessive shearing ofthe melt and causes excessive localized heating which would causethermal degradation.

One solution attempted was to use dual or coaxially spaced melts thatare also concentric. However, even in this instance, there is theadditional problem of the hot melt remaining in the longitudinal groovewhere degradation can begin. The object of the present invention is tominimize any residence time in the accumulator, thereby enhancing itsability to process heat-sensitive materials. In addition, a uniform wallthickness in the parison melt is obtained with this new and improvedmelt feeding system which assures uniform melt distribution around theentire circumference of the parison without the creation of thin spots.

SUMMARY OF THE INVENTION

The present invention relates to an extrusion accumulator head systemwherein a movable plunger is reciprocably mounted inside a stationarycylindrically shaped distributor head that has an inlet portion directlyin line with a melt supply source and a terminal portion that is 180degrees opposite such terminal portion. The distributor head cooperateswith a surrounding wall to direct the melt along a curvilinear channelin cross section from the inlet portion to the terminal portion as wellas along a spiral radial path to the bottom circumferential edge of thedistributor head into an accumulator chamber as defined by the bottomportion of the plunger, the surrounding wall, the distributor head and acentral core or tube. Such plunger recedes to enlarge the accumulatorchamber until sufficient melt is in supply to permit its continuousexpulsion by the plunger to form a parison.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view, partly in cross section, of anextrusion device for use in blow molding machine in accordance with thepresent invention;

FIG. 2 is a front elevational view of an extrusion device as shown inFIG. 1 but with the plunger head in a retracted condition showing areservoir of melt in the accumulator head;

FIG. 3 is a front elevational view of a distributor head shown in FIG.1;

FIG. 4 is a cross-sectional view of the distributor head taken on line4--4 in FIG. 3;

FIG. 5 is a cross-sectional view of the distributor head taken on line5--5 of FIG. 4;

FIG. 6 is a cross-sectional view of the distributor head taken on line6--6 of FIG. 4;

FIG. 7 is a cross-sectional view of the distributor head taken on line7--7 of FIG. 4;

FIG. 8 is a cross-sectional view of a series of profiles of the lowerportion of the distributor head taken on lines A through E in FIG. 4 toillustrate the changing profiles of the flow channel presented by suchdistributor head from the inlet end to the terminal end;

FIG. 9 is a chart illustrating the flowing of melt from the inlet end ofthe distributor head along a plurality of separate flow paths to thebottom end portion of the distributor head;

FIG. 10 is a perspective view of the distributor head showing spiralflow paths for the melt;

FIG. 11 is a front elevational view, partly in cross-section of amodified extrusion device for use in blow molding similar to FIG. 1;

FIG. 12 is a front elevational view of the distributor head shown inFIG. 11; and,

FIG. 13 is a cross-sectional view of the distributor head taken on line13--13 in FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, wherein like reference numerals designatelike or corresponding parts throughout the several views, there is shownin FIG. 1 for an internal plunger design of an extrusion accumulatorhead system a cylindrical support or accumulator body 10. Accumulatorbody 10 of such extrusion head has a central upper bore 12 and a lowerbore 13 coaxial therewith which lower bore 13 has a diameter that issmaller than bore 12. An annular tapered shoulder 15 is formed betweenthe juncture of upper bore 12 with lower bore 13. The accumulator body10 has an inlet bore 16 that communicates a conduit 11 with the upperbore 12. A longitudinally extending distributor head 17 located in upperbore 12 is suitably secured to the accumulator body 10. As viewed inFIG. 1, distributor head 17 has a vertically disposed central bore oropening 18 extending longitudinally therethrough and an annular taperedshoulder 20 at the lower end thereof that cooperates with the taperedshoulder 15 to define a gap or clearance space 21 (FIG. 2) therebetween.Coaxially and concentrically mounted in the longitudinally extending andcentral bore or opening 18 of distributor head 17 is a core tube 23. Thelower end of core tube 23 has a cylindrical mandrel 24 secured theretothat is cooperative with an annular die 25 that is secured to the lowerend of cylindrical support or accumulator body 10 to define an annulargap 26 therebetween, as seen in FIG. 2, that facilitates the formationof the parison. The upper portion of core tube 23 is secured to thepiston rod 19 of a hydraulic cylinder 22 which is secured to a crossbracket 27 that is suitably secured to the stationary cylindricalsupport 10. Hydraulic cylinder 22 is used to move the core tube 23slightly upward from the position shown in FIG. 1 to the position shownin FIG. 2 which creates the gap 26 between the annular die 25 and thelower portion of mandrel 24 to allow for the flow of melt from anaccumulator chamber to be described. In the lower position of core tube23 and mandrel 24 as shown in FIG. 1, mandrel 24 is seated firmlyagainst the tapered inner peripheral surface of annular die 25. As shownin FIGS. 1 and 2, a passageway 28 extends longitudinally through coretube 23 to allow for the controlled passage of pressurized air to helpin the formation of a parison 29 (FIG. 2).

Movably journaled within the longitudinally extending central bore oropening 18 of distributor head 17 is a longitudinally extending annularplunger 30 having a lower tapered annular piston surface 31 and an upperflanged end portion 32 projecting out of the distributor head 17 andsuitably connected to a rod 35 of a piston head 36 in hydraulic cylinder37. Hydraulic cylinder 37 is suitably secured to a cross bracket 38 thatis suitably secured to the cylindrical support 10 as by bolts 39. Asuitable pressurized fluid source is connected to hydraulic cylinder 37to control the reciprocation or movement of piston rod 35 and plunger 30in a manner to be described.

The distributor head 17 in cooperation with the interior wall surface ofbore 12 (of cylindrical support 10) defines a flow channel 40 whoseinlet portion or inlet end 41, as depicted in FIGS. 1 and 3, is directlyacross from the inlet bore 16 in accumulator body 10. One hundred andeighty degrees (180°) opposite the inlet portion or inlet end 41 is aterminal end portion 42 depicted in FIG. 3 as taken across line E ofFIG. 4. In this context numerals 41 and 42 are on the wall portion ofthe distributor head 17 and provide a means to locate the elements andcontour of the distributor head and channel 40. Additional sideelevational cross sections of the distributor head are shown in FIGS. 5,6 and 7 as taken along lines 5--5, 6--6, and 7--7 in FIG. 4. To furtherdefine the contour and configuration of the distributor head 17 is a fewadditional cross-sections are taken along lines A, B, C, D and E of FIG.4 and are illustrated in FIG. 8 by the corresponding letters to show thechanging contour of the channel 40. The maximum depth of flow channel 40at inlet portion 41 is designated as "a" and is the deepest portion ofsuch channel. The maximum depth of the flow channel 40 at the portiontaken on line B is designated "b", while the maximum depth of the flowchannel 40 at the portion taken on line C is "c", with the depth of theflow channel at the portion taken on line D is "d", and portion 42 is"e". In these instances, "a">"b">"c">"d">"e" which stated broadly isthat the maximum depth of the flow from the inlet portion 41 to theterminal end portion 42 is a progressive decrease, i.e., the depthdecreases from the inlet portion to the terminal end portion. In theabove described distributor head, it is to be noted that flow channel 40divides into two flow paths beginning from the inlet portion 41 to theterminal end 42 as depicted by the two arrows L and M in FIG. 4.

In addition to decreasing the depth of the flow channel 40 from theinlet portion 41 to the terminal end portion 42, the flow channel 40also decreases in height from the inlet portion 41 whose height isdesignated h_(a) to the terminal end portion 42 whose height isdesignated h_(e). In prior art distributor heads, a reference is made toan upper diverter channel or groove and below it is a restrictor zone orlower groove. In this type of construction, the upper groove was ineffect a manifold with a large dimension and the lower separate grooveis a restrictor zone referred to as a land. Such land portion acts as amajor resistance to volumetric flow while the manifold acts todistribute the flow to the land, however, the flow in the groove effectsa secondary flow circulation within the groove itself resulting inlaminar folding and mixing which increases flow residence time and leadsto increased chances of thermal degradation. In addition, the gap of theland must in this design be sufficiently restricted to enable themanifold to perform its function of distributing the melt flow, with therestriction again possibly resulting in shear heating of the polymerwhich contributes to thermal degradation. The significance of the flowchannel 40, in distributor head 17, is that the cross sectional shape ofsuch channel is a curvilinear channel having a continuous flow whichintegrates the diverter and restrictor zone to a common single channelto provide a flow rate at the circumferential bottom 51 (FIG. 3) of thedistributor head 17 that is equal at all points around such bottom line.To depict this, consider the flow channel from the inlet end 41 to theterminal end 42 to be flattened and furthermore consider the flowchannel to be symmetric, clockwise and counterclockwise so that only thecounterclockwise portion of the flow channel need be considered asdepicted by FIG. 9. The gap or clearance space between the very bottomof the distributor head 17 and the adjacent inside wall of the lowerbore 13 of the cylindrical support 10 is only uniform whereas theremainder of the gap between the inside wall of the lower bore 13 andthe channel 40 is variable to provide a uniform flow rate along spiralradial lines R₂, R₃, R₄, and R₅, respectively, as depicted by FIG. 10where they all began at inlet 41. Line R₁ is the shortest distance totravel and is along line from inlet 41 in a vertical plane directlydownwardly whereas the remaining flow lines from R₂ to R₅ are radialspiral lines that extend from inlet 41 to the very bottom of thedistributor head 17 as depicted by FIG. 9. The significance of the R₂through R₅ is that the radial spiral lines increase in velocity for eachline from R₂ to R₅ so that all portions of the melt arrive at thecircumferential bottom line of the distributor head at the same time.Another way of considering the radial spiral flow lines as depicted byR₁ through R₅ is to consider them as balancing the flow at the lowestbottom edge of the distribution head by changing the depth of the spiralflow paths differentially along each of these lines, such that thedifferential volumetric flow is the same at each interval along thespiral flow path. To achieve this flow the gap taken (along acircumferential plane that is normal to the central axis of thedistributor head) between the inside wall of the upper bore 12 and theoutside wall of the distributor head (taken on a line that intersectsthe outside wall of the distributor head, which line lies in a planethat passes through the central axis and which plane is normal to thecircumferential plane) increases in dimension from the inlet portion tothe terminal end portion. With these types of increases in dimensions inthe gap, the flow paths of the melt material entering the channel 40 atthe inlet portion 41 takes on a plurality of flow paths as describedabove which are along lines R₁ through R_(n), thus arriving at thecircumferential bottom portion of the distributor head and pushes theplunger 30 upwardly as viewed in FIG. 1 to provide an accumulationthereof in the accumulator chamber 52 (FIG. 2) defined by the plunger30, the lower bore 13 and the core tube 23 until such time that theproper amount is accumulated and thence the plunger 30 is moved rapidlydownward as viewed in FIG. 2 while simultaneously with this action theflow of the melt into channel 40 continues uninterrupted and a parison29 is formed from the extrudate as it passes through the die 25 asmodified by the mandrel 24. In this operation as described above, as themelt is accumulating in the chamber 52 hydraulic cylinder 37 will beactuated to slowly raise the plunger 30 from the position shown in FIG.1 to the position shown in FIG. 2 in cooperation with the force exertedby the melt on plunger 30 until the accumulator chamber has sufficientmelt to form the desired parison at which time the head end of hydrauliccylinder 37 is actuated to force plunger 30 downwardly as hydrauliccylinder 22 is actuated to raise slightly core tube 23 to create a gapor clearance space between mandrel 24 and die 25 to facilitate theformation of the parison.

A modification of the invention is shown in FIGS. 11, 12 and 13 whereina plunger 30' is located circumferentially exteriorly of the distributorhead 17', which head 17' is identical in all aspects to that in thefirst described embodiment except its central longitudinally extendingcentral bore is smaller in diameter since it does not accept a movableplunger. To describe this embodiment, primed numerals are used since thestructure and operation of the structure are substantially identical tothat of the previously described embodiment.

The plunger 30' is annular in shape and mounted in a central bore 80 ofan outer cylindrical support 90. An inner cylindrical support 91 has acircumferentially extending recess on its lower outer periphery forcooperation with the intermediate bore portion 80 of cylindrical support90 to define a chamber or groove 92 within which annular plunger 30' canreciprocate. The upper portions of outer and inner cylindrical supports90 and 91 respectively have flange portions to facilitate theirinterconnect as by bolts 93 to define an integral support, oraccumulator body 10'. The inner cylindrical support 91 has a verticallyextending bore or bores to receive a rod or rods 94 that has one endsecured to the plunger 30' and the other end secured to a piston 95 thatis mounted in hydraulic cylinder 96. Pressurization of the rod end ofcylinder 96 would raise the plunger 30' vertically whereaspressurization of the head end of cylinder 96 would lower the plunger30' as viewed in FIG. 11. The inner bore of inner cylindrical support 91has an upper bore portion 12' and a lower bore portion 13' which arelocated relative to the distributor head 17' in the same manner as inthe first described embodiment, where distributor head 17' is contouredin the same manner as described with respect to distributor head 17 andwhich are further shown in FIGS. 3, 5 through 8 and FIG. 10. The onlydifferences in these heads is that distributor head 17' has a lowerportion 97 that is cylindrical in shape and cooperates with the outercylindrical support 90 to define an accumulator chamber 52' similar infunction to accumulator chamber 52 as described hereinbefore.

A pair of aligned bores 98 and 99 in the inner cylindrical support 91and the outer cylindrical support 90 respectively registers with aninlet end 41' as depicted in FIG. 11. A suitable conduit 100 isconnected to bore 99 to supply hot plasticized melt thereto forprocessing by the cooperative action of the plunger 30' and thedistributor head 17' as described in the first embodiment above. Locatedwithin a lower bore 81 in distributor head 17' is a movable mandrel 24'that is connected by a rod 82 to a piston in hydraulic cylinder 83 whichselectively raises or lowers the mandrel 24'. An annular die 25' issecured to the lower portion of the outer cylindrical support 90 toeither block the flow of melt from chamber 52' or allow its flow whenmandrel 24' is raised to form a parison as is old and well-known in theart.

It will be apparent that, although a specific embodiment and a certainmodification of the invention has been described in detail, theinvention is not limited to the specifically illustrated and describedconstructions since variations may be made without departing from theprinciples of the invention.

What is claimed is:
 1. In an extrusion head for use in a blow molding machine having an extruder for uniformly delivering polymer melt to said extrusion head; said extrusion head having a cylindrical support means with a longitudinally extending central bore and an annular recess with coincident axes; said central bore having an outlet bore portion; a die member mounted at said outlet bore portion; said cylindrical support means having an inlet bore for receiving melt for processing; a distributor head located in the upper portion of said central bore and fixedly secured to said cylindrical support means; said distributor head having a central opening coaxial with said central bore; a plunger journaled in said annular recess; power means connected to said plunger operable to reciprocate said plunger to and from said die member for expressing melt through said die member; said distributor head having a lower circumferentially extending bottom portion; said distributor head having a melt flow channel that extends circumferentially around thereof; said channel having an inlet portion that communicates with said inlet bore of said cylindrical support means; said channel having a terminal end portion diametrically opposite said inlet bore; said channel having an upper curved portion in cross section that merges with a lower linear portion to define a curvilinear channel in cross section; said lower linear portion of said channel terminates at said circumferentially extending bottom portion of said distributor head; said channel is circumferentially extending and descending from said inlet portion to said terminal end portion to direct melt from said channel to that portion of said central bore at the bottom of said plunger to move said plunger away from said die member to accumulate melt therebetween; a gap is formed between said curvilinear channel and the opposite circumferentially extending wall of said central bore; and said gap being dimensioned to provide a uniform radial spiral flow path for melt entering said inlet portion that flows along said distributor head to said lower circumferentially extending bottom portion.
 2. In an extrusion head as set forth in claim 1 wherein the melt flowing radially increases in velocity as the length of said radial flow paths increase in distance traveled form said inlet bore to provide a uniform flow distribution along said entire lower peripherally extending bottom portion of said distributor head.
 3. In an extrusion head as set forth in claim 1 wherein said radial spiral flow path increases in distance traveled from said inlet bore of said cylindrical support means to provide a uniform flow distribution of melt along the entire lower peripherally extending bottom portion of said distributor head.
 4. In an extrusion head as set forth in claim 3 wherein the uppermost portion of said gap as defined by said flow channel decreases in width from said inlet bore to said terminal end portion, and said width is measured along a line that is normal to the axis of said central bore. 